Two small scenes sit side by side here, and the poem lets them quietly echo each other. In the first, rain falls on an evening and feels sad. The speaker compares it to "a letter from a cave," which is a strange, striking image. A cave is dark, hidden and far away, so the rain seems like a message from somewhere deep and silent, one we can't quite read but can sense the mood of.
Then the poem turns to a cat. The speaker wonders what she is seeing in the dark and notices her lifting her head, alert and still. The speaker can't know, and that gap is the point. The cat seems to notice something beyond human sight.
The feeling is thoughtful and hushed, with a touch of loneliness, but also curiosity. Nothing is solved. The rain and the cat both seem to hold mysteries, and the speaker simply sits with them, watching and wondering.